1. Field of the Invention
The present invention relates generally to fuel assemblies for use in nuclear reactors and, more particularly, is concerned with an apparatus and method for fabricating fuel assemblies with reduced residual built-in stresses.
2. Description of the Prior Art
A typical pressurized water nuclear reactor contains a large number of fuel assemblies in its core. Each fuel assembly in the reactor core has the same construction. Basically, a fuel assembly is composed of a bottom nozzle, a top nozzle, an instrumentation tube and pluralities of guide thimbles, fuel rods and grids. The structural skeleton of the fuel assembly is composed of the bottom and top nozzles and the plurality of guide thimbles which extend vertically between the bottom and top nozzles and rigidly interconnect them.
The grids and fuel rods are not structural parts of the fuel assembly but instead are respectively supported directly and indirectly by the guide thimbles. The grids are attached in axially spaced positions along the guide thimbles such that a multiplicity of cells defined by interleaved straps of the respective grids are disposed in vertical alignment. The fuel rods are supported in an organized and transversely spaced array in the vertically aligned cells of the transverse grids by springs and dimples on the straps which extend into the cells.
The grids have short sleeves which extend above and below the interleaved straps for receiving the guide thimbles and providing means for attaching the grids thereto. Typically, concentric portions of the guide thimbles and grid sleeves are deformed or bulged together, such as by plastic expansion, to form mechanical joints therebetween above and below the respective grids. During this process, unequal shortening of the guide thimbles can occur. Unequal shortening of the respective guide thimbles can result in a skewed or bowed fuel assembly skeleton and lock-in of axial stresses between the guide thimbles and in the fuel assembly as a whole.
Consequently, a need exists for an approach to fuel assembly fabrication which will avoid skewing or bowing of the fuel assembly skeleton.